1. Field of the Invention
The invention in general relates to monitoring systems, and particularly to apparatus for detecting undesired metal-to-metal contact between a rotary part and a stationary part of a rotary machine.
2. Description of the Prior Art
In rotating machinery a problem often arises wherein the rotating portion of the machinery contacts the stationary portion thus creating an unwanted and potentially dangerous rubbing condition. For example, in a steam turbine various steam seals exist in order to prevent steam leakage between blade rows as well as to prevent leakage where the rotor penetrates the outer cylinder of the turbine. A mechanical or thermal condition may occur whereby distortion or abnormal operation of the turbine parts may cause one or more of the seals or blades of the turbine to rub thus resulting in a potentially dangerous situation.
The presence of a rub will cause certain abnormal vibrations to occur in the turbine and generally a plurality of vibration sensors are mounted at various points on the turbine system to monitor such vibrations. Other techniques for monitoring rubs include the use of particular sensors for detecting the occurrence of acoustic emissions within the metal parts of the turbine, such acoustic emissions being generated as a result of certain abnormal operating conditions.
A problem arises, however, in the use of acoustic emission or vibration sensing techniques in that it is very often impossible to differentiate between a rub condition as opposed to other conditions relative to the turbine rotor, bearing, etc. which may provide signals similar to those provided by a rub condition.
An improved rub detection arrangement which detects modifications of ground current in the discharge path of such machines is described in copending application Ser. No. 383,400, filed June 1, 1982 and now U.S. Pat. No. 4,502,046.
The present invention, which may be used by itself or in conjunction with prior art techniques to determine the presence of an unwanted rub in rotating machinery, is a modification and improvement over the copending case.